1. Field of the Invention
This invention is related in general to a process for the manufacture of colorants. In particular, it pertains to the vacuum deposition of pigments in a thermoset cross-linked polymer matrix to form a colorant with heat-stable, solvent-resistant, lightfast, dyestuff properties.
2. Description of the Related Art
Colorants are normally understood in the art to include both pigments and dyestuff. Pigments refer mainly to inorganic salts and oxides, such as iron and chromium oxides, which are usually dispersed in crystal or powder form in an application medium. The color properties of the dispersion depends on the particle size and form of the pigment. Pigment colorants tend to be highly durable, heat stable, solvent resistant, lightfast, and migration fast. On the other hand, they also tend to be hard to process and have poor color brilliance and strength.
Dyes (also called dyestuff) are conventionally understood to refer to organic molecules dissolved, as molecular chromophores, in the application medium. Examples are azo dyes, coumarin dyes, and perylene dyes. The color imparted by dyestuff to the resulting solution depends on the electronic properties of the chromophore molecule. Dyestuff colorants tend to have excellent brilliance and color strength, and are typically easy to process, but also have poor durability, poor heat and solvent stability, and high migration.
Because of the contrasting properties of both types of colorants, much work has been done trying to improve the attributes of each class of colorant (see, for example, U.S. Pat. Nos. 4,017,476, 5,470,502, 5,710,197, 5,795,379, 5,871,872) and to combine the advantages of both into a single product (U.S. Pat. Nos. 4,486,237, 5,106,421, and PCT Publication WO 99/40123). These prior-art improvements have often involved binding dyes in a polymeric matrix to achieve greater color durability, improved heat and solvent stability, and to reduce migration as a result of the stable structure provided by the matrix.
A recent publication describes a similar improvement based on a nanocolorant produced by miniemulsion of dyestuff encapsulated into the polymerized structure of an appropriate monomer/co-surfactant/crosslinker mixture. Thorsten, Clemens et al., xe2x80x9cNanocolorantsxe2x80x94Pigments with Dyestuff Properties,xe2x80x9d Polymer
Preprints 2000, 41(1), 24. The resulting colorant has many of the advantages of pigments, especially good migration fastness, and of dyes, especially high brilliance and good dispersability.
All prior-art approaches to the manufacture of colorants with the combined advantages of pigments and dyestuff have been through wet chemistry. These chemical processes produce polymers capable of encapsulating dyes, but typically require relatively long process times and the undesirable step of handling solvents. The range of application of chemical processes is limited by the solubility of the dyes of interest, and the homogeneity of the resulting colorant is dependent on the particle size of the initial dye dispersion. In addition, wet chemistry processes are not suitable for the production of colored metalized films or flakes, for which there exists an ever increasing demand in the market place.
Therefore, there is still a need for a better approach to the process of immobilizing dispersed dyes in a thermoset crosslinked polymer in order to combine the advantages of both pigments and dystuffs within the same colored material. This invention is directed at a process for achieving this objective by flash vapor deposition followed by radiation curing of the various constituents.
The primary objective of this invention is a method for producing a colorant with the combined attributes of pigments and dyestuffs.
Another important objective is a process that produces a perfect, molecular-level dispersion of a dye within a matrix to produce a colored film product or a colored nano-flake for further application in an appropriate carrier medium.
Another goal is a dye-like colorant that is not extractable and therefore is very stable under all normal conditions of use.
Another objective is a process that can be implemented at a high production rate.
Still another objective is a process that can be implemented without the use of solvents.
Another goal is a process that produces a highly homogeneous, defect-free thin film.
Another objective is a process that makes it possible to produce colors within ranges that are not limited by the solubility of the dyes of interest in a solvent medium.
Specifically, a goal of the invention is a process for manufacturing colored metallized films.
Another specific goal is a process for manufacturing colored metal flakes.
Another objective is a procedure that can be implemented utilizing modified prior-art vapor deposition technology.
A final objective is a procedure that can be implemented easily and economically according to the above stated criteria.
Therefore, according to these and other objectives, the present invention consists of selecting a radiation curable monomer or oligomer that is known to produce a polymeric matrix having desirable characteristics for a particular application and a dyestuff that can be flash evaporated in a vacuum deposition chamber. The monomer/oligomer and the dystuff are mixed in a curable formulation and fed into a hot evaporator under vacuum. The blend is flash evaporated through a nozzle and recondensed onto a moving substrate in contact with a temperature-controlled rotating drum. The condensed film is then cured with a high-energy ultraviolet or electron-beam source, or any other high-energy radiation source, to effect cross-linking of the monomer/oligomer.
The resulting film product consists of a crosslinked polymer matrix incorporating dyestuff molecules within its structure. Encapsulating the dyestuff molecules within the crosslinked polymer network dramatically enhances the durability, solvent resistance, heat stability and migration fastness of the product. According to another aspect of the invention, these properties can be further improved by bonding the dyestuff molecules with the crosslinked polymer network, which can be accomplished by judiciously functionalizing the dyestuff with appropriate functional groups designed to polymerize or to react with the curable monomer/oligomer.
The homogeneous or heterogeneous colorant mixture prepared for vacuum deposition is typically liquid and vaporized in conventional manner. If a substantially solid (or waxy) mixture is used, it is preferably granulated and fed into an extruder where the solid particles are heated and gradually melted as they advance toward the extruder""s outlet. A nozzle is used to shape the liquified material into a thin film that is deposited on the revolving hot drum of a conventional vacuum evaporator, whereby the liquid film flashes into a homogeneous vapor that is subsequently cryocondensed to form a thin film in conventional manner.
According to still another aspect of the invention, the solid colorant so produced can be separated from the substrate and crushed to yield colored flakes. These can then be used as colorant ingredients according to conventional practice.
Various other purposes and advantages of the invention will become clear from its description in the specification that follows and from the novel features particularly pointed out in the appended claims. Therefore, to the accomplishment of the objectives described above, this invention consists of the features hereinafter illustrated in the drawings, fully described in the detailed description of the preferred embodiment and particularly pointed out in the claims. However, such drawings and description disclose but one of the various ways in which the invention may be practiced.